1. Field of the Invention
The present invention relates to an IP switch (internet Protocol switch) for transferring packet data in the lower layer of ATM (Asynchronous Transfer Mode), to an interface circuit and an ATM switch used for that IP switch, and to an IP switch network system.
2. Related Art Statement
At present, IP (Internet Protocol) is widely used as a protocol in computer communication in which data is divided into packets on a computer before transmission. According to the 7 layer model of OSI (Open System Interconnection), IP is the protocol which belongs to the network layer, and, to perform communication between different physical networks, communication is realized through routers.
However, it has been generally pointed out that it is difficult to make the IP router fast and to provide it with mass storage because of the characteristics of IP, and accordingly, even when high-speed lower layers are employed, their capacity can not be utilized sufficiently.
Recently, as a solution to these problems, there has been noted a technique called xe2x80x9cIP Switchingxe2x80x9d proposed in Flow Labeled IP: A Connectionless Approach to ATM (Proc. IEEE Infocom, San Francisco, March 1996). This technique is one that introduces the arrangement of ATM (Asynchronous Transfer Mode) known as a high-speed communication technique into the ordinary IP, in an attempt to speed up the IP router.
In the above prior art, the process in the network layer, which is handled by software, is simplified as much as possible, to speed up the process, and retransmission by the cell is not taken into consideration. Accordingly, even if only one cell is lost, the whole frame containing that cell is discarded. As a result, when a virtual connection is changed in the middle of transfer of a frame, that frame is lost from the network.
Conventionally, in ATM, it is provided that a virtual connection is established in advance, and data is transferred on that line, presupposing that the transfer line is not changed in the middle of communication. Thus, there has not been such a problem as a packet being lost owing to change of a transfer line during the course of transfer.
In the IP switching technique, however, since a virtual connection is changed during the course of data transfer, it is possible that such loss of a packet causes a problem. In the following, such an example will be described.
Conventionally, such packet loss has been dealt with by retransmission upon timeout in the network layer. However, this processing usually occurs at the time of a fault such as congestion or link breakage, and too frequent occurrence of this processing may give rise to congestion, or a transfer rate may be lowered by awaiting timeout. Thus, from the viewpoint of quality, it is problematic to use the timeout retransmission technique for such a frequent phenomenon as change of a virtual connection in IP switching.
In particular, in the case that IP switching is used in a public network which employs packet metering as an accounting method, the technique of handling packet loss relying on retransmission in the network layer can not be accepted because double accounting on a user may arise.
An object of the present invention is to provide an IP switch, interface circuit and an ATM switch used for that IP switch, and an IP switch network system, which can prevent loss of a frame caused by change of a virtual connection in the middle of frame transfer.
Another object of the present invention is to provide an IP switch, interface circuit and an ATM switch used for that IP switch, and an IP switch network system, which can avoid retransmission owing to the above-described frame loss caused by change of a virtual connection in the middle of frame transfer, and can avoid double accounting on a user due to retransmission of a packet.
Another object of the present invention is to provide an IP switch, interface circuit and an ATM switch used for that IP switch, and an IP switch network system, which can avoid retransmission owing to the above-described frame loss caused by change of a virtual connection in the middle of frame transfer, and can improve the transfer rate.
In a first invention, when a virtual connection is to be changed, a PT (Payload Type) field of an ATM cell which is currently in the course of transfer is referred to, and the transfer line is kept from being changed until the boundary of the frame is recognized, so that an AAL (ATM Adaptation Layer) frame is protected and packet loss is prevented.
In a second invention, when a virtual connection is to be changed, the multicast function of the ATM switch is employed to transfer a sufficient number of cells for transmission of the maximum length of packets through both transfer lines before and after the change, and thereafter the virtual connection before the change is eliminated, so that an AAL5 (ATM Adaptation Layer Type 5) frame is protected and packet loss is prevented. The xe2x80x9csufficient number of cells for transmission of the maximum length of packetsxe2x80x9d is set in advance, based on the design policy of the network.
In a third invention, when a virtual connection is to be changed, the multicast function of the ATM switch is employed to transfer cells through both transfer lines before and after the change, for a sufficient period of time for transmission of the maximum length of packets, and thereafter the virtual connection before the change is eliminated, so that an AAL5 frame is protected and packet loss is prevented. The xe2x80x9csufficient period of time for transmission of the maximum length of packetsxe2x80x9d is set in advance, based on design policy of the network.